In javascript, what code runs at runtime and what code runs at parsetime?

Question

I'm completely muddled up about what code runs at runtime and what code runs at parsetime. With objects especially, I don't understand what parts of the object run before initialization, what runs at initialization and what runs sometime after.

Since I'm completely confused, I'm not sure how complicated this question is. If it's not just a simple rule or two that answers the question, is there perhaps an article or cheat sheet somewhere? Google is giving me squat.

EDIT: It seems that parsetime is the wrong word. Gareth understood what I meant. I guess I should have formulated the question "In the 2-pass read, what gets read the first pass and what gets read the second pass?" Does that question make more sense?

Answer 1

Unlike C++, it is not possible to run logic in the Javascript parser.

I suspect that you're asking which code runs immediately and which code runs when you create each object instance.

The answer is that any code in a function that you call will only run when you call the function, whereas any code outside of a function will run immediately.

Answer 2

While JavaScript's direct ancestor is Scheme, JavaScript didn't inherit macros, so the answer is fairly simple: there is never any code run during parse time.

Answer 3

A javascript file is run in a 2-pass read. The first pass parses syntax and collects function definitions, and the second pass actually executes the code. This can be seen by noting that the following code works:

foo();

function foo() {
  return 5;
}

but the following doesn't

foo(); // ReferenceError: foo is not defined

foo = function() {
  return 5;
}

However, this isn't really useful to know, as there isn't any execution in the first pass. You can't make use of this feature to change your logic at all.

Answer 4

Not sure what you ask exactly so I'll just share what I know.

JavaScript functions are "pre loaded" and stored in the browser's memory which means that when you have function declared in the very end of the page and code calling it in the very beginning, it will work.

JavaScript global variables, meaning any variable assigned outside of a function, are also preloaded and can be used by code anywhere in the page.

All commands outside of a function will be parsed in the order they appear.

JavaScript doesn't really have "runtime", it can only respond to events or have code executed via global timers. Any other code will be parsed and "forgotten".